How do I mark someone as 'Sick'?

If you click on the shift the employee was due to work you should see a tick box that allows you to Mark as Sick. This will turn the shift into a Sick Leave entry.

Alternatively, you can also delete the shift/move it to your gaps if the shift needs covering. You'll then need to create a new Leave entry by clicking the + button and then Leave and selecting your Sick Leave option.